This invention relates in general to subsurface safety valves and, in particular, to a subsurface safety valve that includes a valve sealing surface and a secondary load bearing surface.
Without limiting the scope of the invention, the background will describe surface controlled, subsurface safety valves, as an example.
Surface controlled, subsurface safety valves are commonly used to shut in oil and gas wells in the event of a failure or hazardous condition at the well surface. Such safety valves are typically fitted into the production tubing and operate to block the flow of formation fluid upwardly therethrough. The subsurface safety valve provides automatic shutoff of production flow in response to a variety of out of range safety conditions that can be sensed or indicated at the surface. For example, the safety conditions include a fire on the platform, a high or low flow line temperature or pressure condition or operator override.
During production, the subsurface safety valve is typically held open by the application of hydraulic fluid pressure conducted to the subsurface safety valve through an auxiliary control conduit which extends along the tubing string within the annulus between the tubing and the well casing. Flapper type subsurface safety valves utilize a closure plate which is actuated by longitudinal movement of a hydraulically actuated, tubular piston. The flapper valve closure plate is maintained in the valve open position by an operator tube which is extended by the application of hydraulic pressure onto the piston. A pump at the surface pressurizes a reservoir which delivers regulated hydraulic control pressure through the control conduit. Hydraulic fluid is pumped into a variable volume pressure chamber and acts against the crown of the piston. When, for example, the production fluid pressure rises above or falls below a preset level, the control pressure is relieved such that the piston and operator tube are retracted to the valve closed position by a return spring. The flapper plate is then rotated to the valve closed position by a torsion spring or tension member.
In conventional subsurface safety valves of the type utilizing an upwardly closing flapper plate, the flapper plate is seated against an annular sealing face, either in metal-to-metal contact or metal against an annular elastomeric seal. In one design, the flapper closure plate has a flat, annular sealing face which is engageable against a flat, annular valve seat ring, with sealing engagement being enhanced by an elastomeric seal ring which is mounted on the valve seat. In another design, the valve seat includes a downwardly facing, conical segment having a sloping sealing surface and the flapper closure plate has a complementary, sloping annular sealing surface which is adapted for surface-to-surface engagement against the conical valve seat surface.
Typically, the flapper closure plate is supported for rotational movement by a hinge assembly which includes a hinge pin and a torsion spring or tension member. It will be appreciated that structural distortion of the flapper valve closure plate, or damage to the hinge assembly which supports the flapper closure plate, can cause misalignment of the respective sealing surfaces, thereby producing a leakage path through the safety valve.
Such misalignment will prevent correct seating and sealing of the flapper closure plate, and formation fluid may escape through the damaged valve, causing waste and pollution. During situations involving damage to the wellhead, the well flow must be shut off completely before repairs can be made and production resumed. Even a small leak through the flapper safety valve in a gas well can cause catastrophic damage.
Attempts have been made to overcome this misalignment problem. For example, one design involves the use of a valve seat and an upwardly closing flapper plate each having a sealing surface with a matched spherical radius of curvature. That is, the valve seat is a concave spherical segment and the sealing surface of the flapper plate is a convex spherical segment. In this arrangement, the spherical radius of curvature of the concave valve seat spherical segment is matched with the spherical radius of curvature of the convex spherical segment which defines the sealing surface on the flapper plate. The matching spherical surfaces are lapped together to provide a metal-to-metal seal along the interface between the nested convex and concave sealing surfaces.
As such, the convex spherical sealing segment of the flapper plate is received in nesting engagement within the concave spherical segment surface of the valve seat, which allows some angular displacement of the flapper plate relative to the valve seat without interrupting surface-to-surface engagement therebetween. Thus, the concave spherical seating surface of the safety valve seat will tolerate a limited amount of misalignment of the flapper plate which might be caused by structural distortion of the closure plate or warping of the hinge assembly.
It has been found, however, the even when using spherical sealing surfaces leakage may occur. Specifically, applications using large diameter tubing and having a low ratio between the outer diameter and the inner diameter of the sealing surfaces, distortion of the flapper closure plate caused by increased loads on the flapper closure plate may result in a loss of the seal. These increased loads are developed as a consequence of using larger safety valves having larger flapper closure plates in larger tubing.
Therefore, a need has arisen for a flapper valve that maintains a seal in a well requiring a large diameter flapper valve having a low ratio between the outer diameter and the inner diameter of the sealing surfaces. A need has also arisen for such a flapper valve that does not experience a loss of the seal in response to distortion of the flapper closure plate caused by the increased loads associated with such designs.
The present invention disclosed herein is a valve comprising a valve closure mechanism that mates with a valve seat, where the valve has enhanced load-bearing capability. The valve of the present invention has separate sealing and load bearing surfaces, and can thus be deployed in a well requiring a large diameter valve having a low ratio between the outer diameter and the inner diameter of the sealing surfaces. The enhanced load-bearing capability of the valve of the present invention is particularly applicable in high pressure situations. Furthermore, the valve of the present invention does not experience a loss of the seal in response to distortion of the valve closure mechanism due to the increased loads on the valve that are associated with such applications.
The valve of the present invention comprises a valve housing, a valve closure member having a sealing surface and a secondary load bearing surface, a valve seat having a valve seat sealing surface, and a secondary load bearing surface that is located on either the valve seat or as part of the valve housing or on both the valve seat and the valve housing. The valve closure mechanism includes a secondary load bearing surface that may be located anywhere on, or formed as an integral part of, the valve closure mechanism. The valve closure mechanism secondary load bearing surface may be either an internal or external shoulder, or one or more internal or external support members, or any combination thereof. The load bearing surface of the valve closure mechanism will mate or engage with a load bearing surface found either on the valve seat or the valve housing, or both.
Should the valve seat include a secondary load bearing surface, the secondary load bearing surface may be either an internal load bearing surface of the valve seat or an external load bearing surface of the valve seat. The secondary load bearing surface of the valve seat may be either an internal or external shoulder, or one or more internal support members, or any combination thereof. A secondary load bearing surface may alternatively be coupled to the valve housing or integrally formed thereon. Should the valve housing include a secondary load bearing surface, the secondary load bearing surface of the valve housing may, for example, be an internal shoulder or one or more internal support members, or any combination thereof.
The valve of the present invention may be a flapper valve. Alternatively, the valve of the present invention may be a gate valve, a ball valve, a poppit, a valve having sliding members, a valve having sleeves, and any other types of valves known in the art. Accordingly, the valve closure member of the valve may be a flapper closure plate, a gate, a ball, a sleeve, a sliding member, or any other structure that forms a seal when mated to or engaged with a corresponding valve seat. Furthermore, a flapper closure plate may be flat or contoured.
In one embodiment, the valve includes a tubular valve housing having a valve chamber. A valve seat is mounted within a housing. The valve seat has a sealing surface and a secondary load bearing surface. A valve closure mechanism is provided as a flapper closure plate having a sealing surface and a secondary load bearing surface. The flapper closure plate is disposed within the valve chamber and rotates between a valve open position, in which the flapper closure plate is removed from the valve seat, and a valve closed position, in which the sealing surface of the flapper closure plate sealingly engages the valve seat sealing surface for preventing flow therethrough. When the flapper closure plate is in the valve closed position, the secondary load bearing surface of the valve seat defines the maximum travel of the flapper closure plate.
In one embodiment of the present invention, the secondary load bearing surface of a valve seat is an internal load bearing shoulder that may be machined as an integral part of the valve seat. In another embodiment, the valve seat may include a seal ring insert that comprises a material having a hardness greater than that of the valve seat. The seal ring insert may be a solid ring. Alternatively, the seal ring may be a machined weld bead. In either case, the seal ring insert forms a portion of the valve seat sealing surface and may serve as an internal load bearing shoulder.
In another embodiment, the secondary load bearing surface of the valve seat is an external load bearing shoulder. In this embodiment, the flapper closure plate includes a ballast member extending from the end of the flapper closure plate opposite the pivot pin, such that the external load bearing shoulder of the valve seat and the ballast member of the flapper closure plate define the maximum travel of the flapper closure plate. The external load bearing shoulder may be used alone or in combination with an internal load bearing shoulder or internal support members, each serving as secondary load bearing surface.